The present invention relates to a throw away insert for milling having a cutting angle of 90.degree. (90.degree. is used herein to refer to angles approximate to 90.degree. ) and a face milling cutter carrying such inserts.
Milling with a cutting angle of 90.degree. (what is called corner cutting) is ordinarily carried out using throw-away inserts having a basically regular triangular shape as shown in FIG. 13 or throw-away inserts having a basically parallelogrammic shape as shown in FIG. 14. Also, as shown in FIG. 15, there is known an insert basically square in shape and having minor cutting edges protruding from the four corners (SANDVIK in Sweden).
These throw-away inserts are mounted on the outer periphery of the cutter body near its tip so that each of their major cutting edges 14 extends parallel to the axis of the cutter or is slightly inclined relative to the cutter axis in such a way that it is closer to the center of the cutter at its rear end. Cutting is done with one of the major cutting edges 14 and a minor cutting edge 15 contiguous to the major cutting edge, the corner angle .alpha. 2 therebetween being about 90.degree. (those shown in FIGS. 13 and 14 ordinarily have a corner angle of 89.degree. and the one shown in FIG. 15 has a corner angle several degrees still smaller). Further, since it is necessary that the cutting edges of these inserts have a positive radial rake and axial rake, their side faces have to have a relief angle. Thus, these inserts are of a positive type in which the side faces intersect the top surface as the rake face at an acute angle.
If the above-described conventional throw-away inserts, are turned over, the positions of the cutting edges are of reversed even if they are a negative type. Thus, cutting edges can be formed only on one side. This means that the numbers of cutting edge corners which can be formed on the inserts of FIGS. 14 and 13 are limited to 2 and 3, respectively.
The insert shown in FIG. 13 is used more extensively than the insert of FIG. 14 because the former has one more cutting edge corner and thus is economically advantageous. But this insert, having a basically regular triangular shape, possesses insufficient strength because its corners are thin. It is possible to increase its strength to a level comparable to other inserts by increasing the widths of the minor cutting edges 15. But wider edges 15 mean that the distance L between the center of the insert and the minor cutting edges is small. Thus, the support points X, Y and Z are arranged in an unbalanced manner, so that the insert cannot be supported stably. Thus, it is difficult to increase the strength by widening the minor cutting edges.
On the other hand, the insert of FIG. 15 has sufficiently high edge strength and has four cutting edge corners. But this insert has a major drawback in that the cutting amount is limited to d due to the interference by the adjacent minor cutting edges.
Further, since these inserts have side faces inclined (with respect to a plane perpendicular to the top surface) at an angle of 11.degree.-20.degree. or greater, side faces cannot be supported stably on the support surface. Thus, it is difficult to mount the cutter accurately.
A conventional face milling cutter cannot dispose of chips forcibly and thus the curling diameter of chips tends to be large. Thus, in order to curl such chips, as shown in FIG. 16, it was necessary to provide a concave chip pocket 4 in each back metal portion 3 defined between adjacent gullets 2 formed in a cutter body 1 of a conventional face milling cutter. Such pockets lower the strength of the cutter body, so that the body is more likely to be deformed. This may lead to reduced machining accuracy or chattering during cutting.
The body surface is subjected to black oxide surface treatment (coloring with caustic soda, what is referred to as a black oxide finish). But this treatment cannot sufficiently increase the corrosion resistance or the surface hardness. Thus, the body can be damaged easily by contact with chips.